1. Technical Field
The invention relates to methods and systems for controlling the nominal storage and release times used in connection with an emission control device to facilitate xe2x80x9clean-burnxe2x80x9d operation of an internal combustion engine.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust gas that includes a variety of constituents, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituents are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more exhaust gas constituents, such as Nox, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio (a xe2x80x9cleanxe2x80x9d engine operating condition), for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select exhaust gas constituents, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NO, storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device for a selected exhaust gas constituent is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected exhaust gas constituent may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-retaining efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
The amount of the selected constituent gas that is actually stored in a given emission control device during vehicle operation depends on the concentration of the selected constituent gas in the engine feedgas, the exhaust flow rate, the ambient humidity, the device temperature, and other variables including the xe2x80x9cpoisoningxe2x80x9d of the device with certain other constituents of the exhaust gas. For example, when an internal combustion engine is operated using a fuel containing sulfur, the prior art teaches that sulfur may be stored in the device and may correlatively cause a decrease in both the device""s absolute capacity to store the selected exhaust gas constituent, and the device""s instantaneous constituent-storing efficiency. When such device sulfation exceeds a critical level, the stored SOx must be xe2x80x9cburned offxe2x80x9d or released during a desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of SOx.
Thus, it will be appreciated that both the device capacity to store the selected exhaust gas constituent, and the actual quantity of the selected constituent stored in the device, are complex functions of many variables that prior art accumulation-model-based systems do not take into account. The inventors herein have recognized a need for a method and system for controlling an internal combustion engine whose exhaust gas is received by an emission control device which can more accurately determine the amount of the selected exhaust gas constituent, such as NOx, stored in an emission control device during lean engine operation and which, in response, can more closely regulate device fill and purge times to optimize tailpipe emissions.
Under the invention, a method is provided for controlling the fill and purge cycle of an emission control device disposed in an exhaust treatment system for an internal combustion engine. Under the invention, values representing an instantaneous rate at which a selected constituent of the engine-generated exhaust gas, such as NOx, is stored in the device, and the instantaneous capacity of the device to store the selected constituent, are determined as a function of a calculated value representing an amount of SOx which has been accumulated in the device since an immediately prior desulfation event. More specifically, in a preferred embodiment, the calculated value representing the amount of accumulated SOx is determined as a function of the instantaneous fuel flow rate during lean and stoichiometric engine operating conditions, preferably further adjusted to reflect the effects of instantaneous air-fuel ratio and instantaneous device temperature on the accumulation of SOx in the device.
In accordance with another feature of the invention, in a preferred embodiment, the calculated value representing the amount of accumulated SOx is used to schedule a device-regeneration or xe2x80x9cdesulfationxe2x80x9d event. Specifically, the value is preferably compared with a predetermined threshold value, with a desulfating engine operating condition being selected when the calculated accumulated SOx value exceeds the predetermined threshold value.
In accordance with another feature of the invention, the values representing the instantaneous storage rate for the selected constituent in the device, and the instantaneous storage capacity, are further determined as a function of a determined value representing a permanent reduction in the constituent storage capacity of the device due to thermal effects and xe2x80x9cpenetratedxe2x80x9d or diffused sulfur which cannot otherwise be purged during a nominal device-desulfation event.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.